1. Field of the Invention
This invention relates to a method of processing the gradation of a radiation image used for medical diagnosis and apparatus for carrying the method. This invention particularly relates to an image gradation processing method and apparatus for processing the gradation of a radiation image which is read out from a stimulable phosphor carrying the radiation image stored therein and reproduced into a visible image.
2. Description of the Prior Art
For medical diagnosis, there are generally used X-ray films for taking radiographs. Since the X-rays do harm to the human body, it is impossible or undesirable from the viewpoint of safety to expose the human body to X-rays of high dose. Therefore, it is desirable that the necessary information in the radiograph can be obtained by exposing the human body only once to the X-rays of comparatively low dose. On the other hand, radiographs should preferably have both a wide exposure latitude and high image quality of high contrast, high sharpness, low noise etc. for viewing and diagnosis purposes. Unfortunately, since the conventional radiography is designed so as to satisfy all the required conditions to some extent, the range of recording density or the ability to record various levels of information and the image quality are both insufficient and neither of these properties are completely satisfied.
In view of the above circumstances, it has been proposed in U.S. Pat. No. 3,859,527 to have a radiation passing through an object be absorbed by a stimulable phosphor and then stimulate the phosphor by light energy to cause the phosphor to emit the radiation energy stored therein as light so that the emitted light is detected and converted to an electric signal used for reproducing a visual image.
This radiation image system using the stimulable phosphor is advantageous over the conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing and diagnosis purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the phosphor to the radiation by converting the emitted light to an electric signal and changing the level of the electric signal to a desirable level corresponding to the desirable optical density of the image reproduced on a photographic film or the like. This is very advantageous in practical use. For instance, when there are differences in exposure among a number of radiation images or there are over- or under-exposure images in a number of radiation images, these images can be processed to have the same level of optical density finally. Accordingly, the mistakes in the step of exposure can be corrected easily. Further, the optical density of the finally obtained image can be freely selected simply by changing the level of the electric signal used for reproducing the image, and accordingly, it is possible to easily obtain an image having a desirable density for any kind of image. In other words, in the radiation image of the human body the desirable density is different depending upon the kind of the image. In one kind or part of the human body a high density is desirable for obtaining high diagnostic efficiency and accuracy and in another kind or part a low density is desirable. In the conventional radiography, the exposure is controlled to obtain the desirable density for the various kinds of images. In this sense, the above-mentioned system utilizing the stimulable phosphor and a gradation processing means is very advantageous. Furthermore, in the conventional radiography a number of films of different sensitivity are prepared to be accommodated to a number of intensifying screens of different sensitivity. In the above-mentioned system, however, it is unnecessary to prepare a number of films of different sensitivity since one kind of film can be accommodated to various conditions of exposure and various sensitivities of the intensifying screen by later changing the level of the electric signal as desired.
As mentioned above, in the radiation image recording system using a stimulable phosphor the image information having a very wide range of level corresponding to the very wide range of exposure is once stored in the stimulable phosphor and is then read out and converted to an electric signal and finally converted to a visible image after processing the electric signal as desired. Therefore, the optical density of the finally obtained visible image or reproduced image can be controlled to the level desirable for diagnosis purposes. Thus, a radiation image having high diagnostic efficiency and accuracy can be obtained.
In order to put the above-mentioned radiation image recording system into practice, the aforesaid processing of the electric signal should be quantitatively standardized for all kinds of radiation images. The standardization should be conducted from the viewpoint of enhancing the diagnostic efficiency and accuracy (the level of easiness for diagnosis or adaptability to diagnosis). The diagnostic efficiency and accuracy are not simply enhanced by simply making so-called good image for the viewpoint of the ordinary image quality factors such as sharpness, granularity and contrast. Rather than these factors, the diagnostic efficiency and accuracy are influenced by other complex factors such as reference with the normal shade, reference with the anatomical structure and utilization of other diagnostic view or records. Thus, the standardization to quantitatively standardize the process of the electric signal used for reproducing the radiation image to enhance the diagnostic efficiency and accuracy is not yet clarified or established.